Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity.
The advent of solid state diodes that emit an infrared laser beam has expanded the useful applications of infrared-sensitive photographic elements. These include making prints from computer assisted tomography scanners, various graphic arts products that are exposed by diode lasers, and infrared-sensitive false color-sensitized photographic materials as described in U.S. Pat. No. 4,619,892 of Simpson et al.
False color infrared-sensitive photographic elements generally have a first layer that is sensitive to infrared radiation and one other layer that is sensitive to red or infrared radiation. This other layer has a maximum sensitivity at a wavelength different from the first infrared-sensitive layer. One problem encountered by such photographic elements is poor image separation between the different layers. This is due to unwanted sensitivity of one layer to radiation that is intended to expose the other layer(s) caused by overlap of spectral sensitization ranges of the sensitizing dyes.
The above-referenced U.S. Pat. No. 4,619,892 describes infrared sensitizing dyes such as: ##STR2## where Z is a heterocycle of the type useful in cyanine dyes (e.g., benzothiazole) and R is alkyl. The '892 patent address the problem of image separation with a number of well-known techniques, such as speed differences between the various silver halide emulsion layers, filters layers between the silver halide emulsion layers, or combinations thereof. Such techniques, however, are limited in the amount of improvement in image separation that is provided, due to the inherent overlap in the wavelengths of spectral sensitization imparted by the dyes.